Hospitals, nursing homes, and other wearer care facilities typically include patient monitoring devices at one or more bedsides in the facility. Patient monitoring devices generally include sensors, processing equipment, and displays for obtaining and analyzing a medical wearer's physiological parameters such as blood oxygen saturation level, respiratory rate, and the like. Clinicians, including doctors, nurses, and other users, use the physiological parameters obtained from patient monitors to diagnose illnesses and to prescribe treatments. Clinicians also use the physiological parameters to monitor wearers during various clinical situations to determine whether to increase the level of medical care given to wearers. Additionally, monitoring equipment is often used in corporate care facilities, fitness facilities, recreational and home care applications, as well as mobile or other emergency care environments.
Blood pressure (which can refer to diastolic pressure, systolic pressure, and/or some combination or mathematical representation of same) considered one of the principal vital signs, is one example of a physiological parameter that can be monitored. Blood Pressure monitoring is an important indicator of a wearer's cardiovascular status. Many devices allow blood pressure to be measured by manual or digital sphygmomanometer systems that utilize an inflatable cuff applied to a person's arm. The term “sphygmomanometer” is meant to receive its ordinary broad meaning known to an artisan to include devices used to measure blood pressure. These devices often include an inflatable cuff to restrict blood flow and a device capable of measuring the pressure. Other device(s) are used to determine at what pressure blood flow is just starting and at what pressure it is just unimpeded, commonly referred to as “systolic” and “diastolic,” respectively. The term “systolic blood pressure” is meant to receive its ordinary broad meaning known to an artisan to include the pressure exerted on the bloodstream by the heart when it contracts, forcing blood from the ventricles of the heart into the pulmonary artery and the aorta. The term “diastolic blood pressure” is meant to receive its ordinary broad meaning known to an artisan to include the pressure in the bloodstream when the heart relaxes and dilates, filling with blood.
In a typical pressure monitoring system, a hand actuated pump or an electric motor inflates the inflatable cuff to a pressure level at or above the expected systolic pressure of the wearer and high enough to occlude an artery. Automated or motorized blood pressure monitoring systems use a motor or pump to inflate the inflatable cuff, while manual blood pressure monitors typically use an inflation bulb. As the air from the inflatable cuff is slowly released, the wearer's blood pressure can be determined by detecting Korotkoff sounds using a stethoscope or other detection device placed over an artery.
Alternatively, digital sphygmomanometers compute diastolic and systolic pressure as the inflatable cuff deflates based on the oscillations observed by a pressure sensor on the cuff. For example, some digital sphygmomanometers calculate the systolic blood pressure as the pressure at which the oscillations become detectable and the diastolic pressure as the pressure at which the oscillations are no longer detectable. Other digital sphygmomanometers calculate the mean arterial pressure first (the pressure on the cuff at which the oscillations have the maximum amplitude). The diastolic and systolic pressures are then calculated based on their fractional relationship with the mean arterial pressure. Other algorithms are used, such as identifying the change in slope of the amplitude of the pressure fluctuations to calculate the diastolic pressure.
As mentioned above, the foregoing methods of determining blood pressure include inflating the cuff to a pressure high enough to occlude an artery and then determining blood pressure during deflation of the inflatable cuff. Occluding the artery and then determining blood pressure during deflation can have a number of drawbacks. For example, inflating the inflatable cuff to a pressure higher than systolic pressure can cause pain and discomfort to the wearer. Other adverse effects can include limb edema, venous stasis, peripheral neuropathy, etc, or simply wearer interruption. In addition, as the artery is completely occluded prior to each measurement, sufficient time must elapse between measurements to ensure accurate results. Furthermore, manual systems make it difficult to measure blood pressure during inflation of the inflatable cuff due to the difficult of inflating the inflatable cuff at an approximately constant rate using an inflation bulb.
Digital blood pressure monitors can have additional drawbacks. The motors used to pump gas into the cuff are often noisy and can disturb wearers at rest. This is especially problematic in recovery situations. In addition to auditory noise in automated or motorized systems, the motors can cause electrical noise in sensor signals making signal processing used to identify reference points for blood pressure detection unreliable and difficult. Furthermore, portable motorized blood pressure monitors require a significant amount of power to produce the air pressure required to inflate the cuff. Since batteries are often used to provide power, designers often use large batteries and/or batteries that frequently need to be recharged or replaced. When a large batter is chosen, its size often offsets the goals of portability as an appropriate housing becomes more cumbersome and less convenient.